Cytosolic thymidine kinase 1 (TK1) catalyzes the reaction of thymidine and ATP to form TMP (thymidine 5'-monophosphate) and ADP. TK1 has been purified from human spleen and from cultured cells. The enzyme is active as a homotetramer, and phosphorylates thymidine and deoxyuridine using ATP as a phosphate donor in vitro (Sherley and Kelly 1988a; Munch-Petersen et al. 1991; Birringer et al. 2005). Divalent cations are required for enzyme activity (Mg++ is preferred) (Lee and Cheng 1986), and ATP stabilizes the tetrameric form of the enzyme (Munch-Petersen et al. 1993). In cells, enzyme activity is high during S phase of the cell cycle and low otherwise, correlated with intracellular levels of thymidine kinase 1 protein (Sherley and Kelly 1988b), and consistent with a requirement for TMP synthesis in normal cells only as part of DNA replication.